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Physicists Produce Antineutrino Map Of The World

Physicists Produce Antineutrino Map Of The World

If physicists want to measure the antineutrinos produced inside the Earth, they’ll need to avoid the antineutrinos produced by nuclear reactors on the surface.

One of the great mysteries in geophysics is the nature of the interior of the Earth. We have a reasonable idea of the planet’s general internal structure, various techniques for imaging structures beneath the surface and a rough idea of how much heat is generated from the Earth’s interior.

But geophysicists want much more. One question, in particular, is exactly where heat is generated inside the Earth.

Physicists know that almost all of this heat is generated by the decay of radioactive elements such as potassium-40, thorium-232 and uranium-238. But how are these elements distributed and how much heat does each contribute?

In the next few years, geophysicists hope to get some detailed answers to this question thanks to the emerging science of neutrino geophysics. The radioactive decay inside the Earth produces subatomic particles known as antineutrinos. So an experiment that measures the antineutrinos coming out of the Earth should provide a detailed picture of the distribution of these elements within it.

That’s the theory. In practice, this is a tricky task for two reasons. The first is that antineutrinos are famously hard to detect because they interact so weakly with ordinary matter.

In recent years, however, physicists have made great strides in building detectors that can spot them, particularly those generated beyond our shores by the Sun, for example. And in the last couple of years, two of these experiments have spotted the first geophysical antineutrinos from inside the Earth.

That’s triggered huge interest from physicists who want to repeat the experiments in other parts of the world. That brings us to the second problem.

This is that nuclear reactors also produce antineutrinos and so generate a strong background signal that can swamp the signal from inside the planet.

What’s needed, of course, is a global map showing where the background antineutrino signal is strongest. And today, we get it thanks to the work of Barbara Ricci at the University of Ferrara in Italy and a few pals who have made the most detailed map of reactor antineutrinos ever produced.

Their approach is straightforward. These guys take data from the International Agency of Atomic Energy giving the thermal power of every reactor on the planet. That gives Ricci and co a good idea of how many antineutrinos each reactor core produces and allowed them to calculate the antineutrino flux all over the world.

Next, they considered an antineutrino detector containing 10^32 protons and calculated how many antineutrinos from reactor cores this device would spot over the course of a year.

Finally, they plotted their results for the entire planet showing the areas where the background signal for any antineutrino detector would be highest. You can see the map here ( 1 TNU= 1 event/yr/10^32 detector protons ).

Clearly, there are some parts of the world that are ‘quieter’ and so much better for detecting geophysical antineutrinos than other places. For example, physicists are already planning to build antineutrino detectors in Hawaii and in Curacao off the coast of Venezuela. “Hawaii and Curacao are wonderful places for geo-neutrino studies due to their position far away from any nuclear plants of the world,” say Ricci and co.

A Canadian experiment called the Sudbury Neutrino Observatory + is also currently gathering data and appears to be in a reasonable spot. “In the near future, the SNO+ experiment, with a quite reasonable ratio [of reactor antineutrinos to geophysical antineutrinos], will provide more information about Earth’s interior ,” they say.

And Japan has recently become a better place to hunt for geophysical antineutrinos because of the shutdown of the country’s nuclear industry following safety concerns raised after the 2011 Tohoku earthquake and tsunami. As a result, the detector there, Kamioka, has become a more suitable site for detecting geophysical neutrinos, say Ricci and co.

Other locations are not so good, however. One of these is the Fréjus Underground Laboratory in south-east France near the border with Italy, which is currently being studied as a possible location for a European antineutrino experiment.

A cursory look at the map reveals that this location could be fraught with difficulty. “A new European geo-neutrino detector located at Frejus Laboratory requires a detailed knowledge of close by reactors,” say Ricci and co diplomatically. They point out that a better option for Europe might be the Pyhäsalmi Mine in Finland.

Whatever locations are eventually chosen, geophysicists are set for a data bonanza. In the next few years, as these experiments are built and begin to produce data, geophysicists are going to have an entirely new picture of the processes at work in the Earth’s interior.